Interlaminar Fracture Characterization of a Carbon- Epoxy Composite in Pure Mode Ii

The interlaminar fracture toughness in pure mode II (GIIc) of a Carbon-Fibre Reinforced Plastic (CFRP) composite is characterized experimentally and numerically in this work, using the End-Notched Flexure (ENF) fracture characterization test. The value of GIIc was extracted by a new data reduction scheme avoiding the crack length measurement, named Compliance-Based Beam Method (CBBM). This method eliminates the crack measurement errors, which can be nonnegligible, and reflect on the accuracy of the fracture energy calculations. Moreover, it accounts for the Fracture Process Zone (FPZ) effects. A numerical study using the Finite Element Method (FEM) and a triangular cohesive damage model, implemented within interface finite elements and based on the indirect use of Fracture Mechanics, was performed to evaluate the suitability of the CBBM to obtain GIIc. This was performed comparing the values of GIIc inputted in the numerical models with the ones resulting from the application of the CBBM to the numerical load-displacement (P-δ) curve. In this numerical study, the Compliance Calibration Method (CCM) was also used to extract GIIc, for comparison purposes. Introduction Advanced composite materials such as CFRP have recently come to the fore owing to a set of interesting characteristics over the conventional engineering materials such as aluminium or steel. In fact, these high performance composites are being increasingly used in structures requiring high specific strength and stiffness, such as in the automotive, marine, military, aeronautical and aerospace industries. Unidirectional CFRP composites are also ideal for reinforcement purposes, not changing significantly the structures stiffness, and are corrosion resistant. However, it is known that layered composites are prone to develop internal damage, like matrix cracking and delamination, which can be particularly dangerous for the structural stability, often leading to premature catastrophic failures. The internal damage is not easily detectable, which increases the associated risks. In the majority of real applications, matrix cracking and delamination are associated and constitute a typical damage mechanism of composites, especially when structures are submitted to bending loads [ 12]. Thus, the interlaminar fracture toughness characterization of CFRP composites acquires special relevancy in the context of the strength prediction of CFRP structures, and also to study the strength behaviour of adhesively-bonded joints or repairs, since in many occasions damage in these assemblies grows between composite layers, due to a typically smaller strength than the adhesive layer [ 34]. Fracture characterization under pure mode II is usually accomplished using the ENF fracture characterization test [ 5]. The main advantages of this experimental test are its simplicity and the possibility to obtain GIIc mathematically using the beam theory [ 6]. However, problems related to unstable crack growth and to crack monitoring during propagation are yet not well solved [ 7]. Actually, in mode II fracture characterization tests, the crack tends to close due to the applied load, which hinders a clear visualization of its tip, difficulting the crack monitoring during propagation required for the classical data reduction schemes, based on beam theory analysis and compliance calibration. On the other hand, a FPZ ahead of the crack tip exists under mode II loading, which affects the measured fracture energy. Consequently, its influence should be taken into account, which does not occur when a real crack length is used in the selected data reduction scheme. Truss et al. [ 8] studied the interlaminar and intralaminar fracture toughness of a continuous and an aligned discontinuous carbon fibre/epoxy composite using the Compact Tension and Double Cantilever Beam tests. Extensive fibre bridging was found in both interlaminar and intralaminar tests. The authors concluded that the misalignment in the discontinuous fibre samples increases the fracture toughness at initiation and propagation. In this work, the value of GIIc of a CFRP laminate is obtained by a new data reduction scheme not requiring the crack length measurement, the CBBM. This method eliminates the crack measurement errors, which can be non-negligible, and reflect on the accuracy of the fracture energy calculations. Moreover, it accounts for the FPZ effects. A comparative study was also performed to validate the CBBM. This analysis employed the FEM and a triangular cohesive damage model, based on the indirect use of Fracture Mechanics and implemented within interface finite elements, to evaluate the suitability of the CBBM to obtain the value of GIIc. This was accomplished comparing the values of GIIc inputted in the numerical models with the values obtained applying the CBBM to the numerical P-δ curves. The CCM was also applied to the FEM results, for comparison of the results. Experimental work The geometry and dimensions of the ENF specimens are shown in Fig. 1 (a). The specimens consisted of unidirectional 0o lay-ups of carbon/epoxy prepreg (SEAL Texipreg HS 160 RM) with 24 plies of 0.15 mm of unit thickness, whose ply elastic orthotropic properties are presented in Table 1. The initial crack was made introducing a thin Teflon film (thickness of 25 μm) during lay-up. Curing of the laminate was accomplished in a press during one hour at 130oC and 4 bar pressure. Six specimens were tested using an INSTRON 5848 electro-mechanical Microtester equipped with a 2kN load cell, at room temperature and under displacement control (2 mm/min). The P-δ curve was registered during the test. Fig. 1 (b) shows the test setup. a) b) Fig. 1 – ENF specimen geometry (a) and test setup (b). Table 1 –Elastic orthotropic properties of a unidirectional CFRP ply aligned in direction x [ 9]. Ex=1.09E+05 MPa νxy=0.342 Gxy=4315 MPa Ey=8819 MPa νxz=0.342 Gxz=4315 MPa Ez=8819 MPa νyz=0.380 Gyz=3200 MPa Data reduction schemes The classical data reduction schemes to obtain GIIc are usually based on compliance calibration or beam theory. The Compliance Calibration Method (CCM) is based on the Irwin-Kies equation [ 10]